1. Field of the Invention
The present invention relates to the field of electrical devices and in particular to a portable fluorescent drop-light including multiple lamps connected in parallel with switches to allow the individual lamps to be independently turned on or off and a ballast circuit comprising solid-state devices to drive and protect the multiple lamps with or without integral starters.
2. Description of the Related Art
Circumstances often arise around the home and in shops in which a person wishes to illuminate an area that is not adequately illuminated by the installed light fixtures. Flashlights are sometimes used in these circumstances, however as they are powered by batteries, they typically do not offer the illuminating power of hardwired lights. In addition the batteries are of limited lifespan and the light from a flashlight is typically focussed whereas the desire is often for more diffused illumination.
Drop-lights are powered by line supply (120 VAC, 60 Hz) but are portable, hand-held assemblies to thereby allow the user to provide light where the installed light fixtures do not provide coverage. Drop-lights are powered from wall receptacles via a power cord and are not subject to the power and life span limitations of batteries. Drop-lights are typically provided with a hook to allow them to be hung in position and typically employ an incandescent light bulb to provide the illumination.
Incandescent light bulbs consist of a glass bulb enclosing a tungsten filament in an inert gas or highly evacuated atmosphere. When an electric current passes through the filament, the filament heats up to a high temperature at which point it becomes incandescent and provides illumination. The atmosphere surrounding the filament is either an inert gas or evacuated to near vacuum in order to minimize deleterious reactions of the tungsten filament at elevated operating temperatures. Tungsten is used because it has an extremely high melting point and can maintain the high temperatures necessary for incandescence with minimal deterioration.
Incandescent light bulbs are a well-developed technology and are economical to purchase. However, incandescent light bulbs are not particularly economical to operate. Incandescent light bulbs are not very efficient at converting electrical energy to light energy. A typical incandescent lamp produces approximately 20 lumens per watt of electrical power consumed. Much of the electrical energy is converted to heat. The tungsten filament in a typical incandescent bulb operates at approximately 1500–2700K. A 75–100 W incandescent bulb will get too hot to touch during operation.
The high operating temperatures of incandescent light bulbs present some particular drawbacks when used in drop-lights. The heat emitted by the bulb itself makes them uncomfortable to have in close proximity to a person for an extended period. Also, the high operating temperature and fine structure of the tungsten filaments makes them susceptible to breaking if they are jarred in use. Since a drop-light is portable, users will generally move them about during use to place them in the optimal position for illuminating their work. This movement while the filament is hot stresses the filament and an incandescent light bulb in a drop-light will generally have a much shorter life span than it would in a stationary light fixture. Finally, the bulb enclosure is typically a thin glass bulb susceptible to breakage and, if the bulb breaks, the hot tungsten filament is exposed. It can be easily appreciated that this could present an extreme fire hazard if the user is working around flammable materials with relatively low flash points.
Gas discharge lamps are an alternative means of converting electrical energy to light energy that offer significant advantages to incandescent lighting, particularly in drop-light applications. Gas discharge or fluorescent lamps, consist of a gas or vapor filled tube that is provided with electrodes at either end of the tube. When a high enough voltage is applied between the electrodes, the gas inside the tube partially ionizes and undergoes a phase change to the plasma state. The plasma state gas is conductive and serves as a conductor for an electric arc between the electrodes. As current passes between the electrodes, electrons collide with gas molecules within the tube. When an electron collides with a gas atom, an electron in the atom's shell is boosted to a higher energy level. This higher energy level is not a stable condition and when the electron falls back to its normal energy level, a photon of light is emitted. Thus the gas luminesces and gives off the characteristic “fluorescent” light.
Fluorescent lights convert more of the supplied electrical energy to light energy than incandescent lights do and fluorescent lights operate at lower external temperatures. The light emitting medium in fluorescent lights is a gas that cannot “break” like the thin tungsten filament in an incandescent light bulb. While a broken fluorescent light tube does present some health hazards, there is not the tungsten filament operating at 1500–2700K to present an ignition source for flammable materials.
Typically, a higher voltage is required to initiate the plasma state than is required to maintain the plasma state and the luminescence. Also, once the gas starts becoming a plasma, the effective resistance between the electrodes becomes negative. More of the gas will become plasma and the fluorescent light tube will tend to draw more and more current even though the applied voltage stays the same. If this phenomenon is allowed to continue unabated, the tube will overdraw and burn itself out. The excessive current draw can also damage the supply circuit.
Typical practice in the art is to provide a damping circuit that can function both to “spark” the gas tube into operation and also to limit the current that is supplied to the tube. This damping has typically been accomplished with a ballast that primarily consists of a large transformer/inductor. The ballast functions both to transform the supplied line voltage (which is an AC voltage) to a high enough potential to ensure that the fluorescent tube “lights off” and also to provide a high enough inductance in the supply circuit to prevent the fluorescent light tube from overdrawing and damaging itself or the supply circuit during operation.
Alternatively, fluorescent bulbs are available that include integral “starting” ability. The ballast circuits for such lamps provide an appropriate voltage to the lamps and the lamps start themselves. The ballast circuit then regulates the current draw in a similar manner to that previously described for non-self starting bulbs.
The requirements of a ballast for typical line supplies (120 VAC, 60 Hz) are such that the wire gauge and number of turns in the coils and size of the magnetic coupling core result in ballast's that are relatively bulky and heavy. To overcome the size and weight issue, previous art has been developed that incorporates solid state devices into circuit designs to allow similar ballasting functions to be performed in much more compact and lighter assemblies. This has enabled lightweight hand-held fluorescent drop-lights to be brought to market.
The typical design for hand-held fluorescent drop-lights powered by line supplies uses either a twin tube standard bulb or a quad tube standard bulb. The twin tubes have the advantage that they are less expensive and more readily available. However, the twin tubes are of approximately half the power (13 W) and light output of the quad tube lamps. The quad tube design offers approximately twice the power (27 W) and illumination of a single twin tube, however the quad tube bulb is more expensive to produce and purchase and is not as readily available. The size and weight restrictions of drop-light applications generally preclude employing multiple independent lamp circuits as the additional ballast circuits required for each lamp occupy too much space and are too heavy to make such a drop-light handy in use.
From the foregoing, it can be seen that there is a continuing need for a portable hand-held lamp that is efficient to operate and does not operate at excessively high temperatures. The lamp should be sturdy and durable and replacement bulbs should be inexpensive and readily available. It would be a further desirable feature to provide variable illumination and as light-weight and compact a lamp as possible.